Add Optional Support for ESP-IDF Bundled CA Certificates

[hoeken]

This PR adds optional support for using the ESP-IDF CA certificate bundle when running on Arduino-ESP32 3.x (ESP-IDF 5.x).

✅ New Feature: Bundled Certificate Support

A new configuration field enables usage of the built-in CA bundle:

esp32FOTA.useBundledCerts();

or

cfg.use_bundled_certs = true;

When enabled:

• esp32FOTA attempts to load the ESP-IDF CA bundle via setCACertBundle()
• A weak-symbol check detects whether the bundle is actually linked
• If present, HTTPS requests use the full CA bundle
• If not present, esp32FOTA safely falls back to the existing TLS behavior

✅ Fully Backwards Compatible

• No changes to existing behavior unless the new option is explicitly enabled
• Existing root_ca and unsafe modes continue to function normally
• Compatible with both Arduino-ESP32 2.x and 3.x
• Compatible with Arduino IDE and PlatformIO
• No build-time dependency on the CA bundle unless the user links it

✅ Runtime-Safe Detection

Weak symbol declarations prevent build failures when the CA bundle is not available.
If the symbols are missing, esp32FOTA logs a message and falls back gracefully.

Summary

This PR introduces a secure, modern HTTPS configuration path using the ESP-IDF root certificate store, without breaking existing setups or requiring code changes for current users.

[tobozo]

thanks !